Preparation of field for rice-fish culture

Good preparation is very important in order to succeed in rice-fish culture. Every farmer must be able to:

hold enough water over a large enough area for enough time to produce enough fish; and

prevent serious flooding of the dikes and other boundaries of his ricefield.

Having a satisfactory water situation in the field is a key factor in the technology; this cannot be achieved if preparation is poor. In field preparation, there are four main things to consider: field size and shape, dikes, refuges and drains.

Field size and shape

1. How much land does the farmer own? If the farmer does not own the land and the landlord is agreeable, how big an area does the landlord want to try?

2. Topography and slope will greatly affect field size and shape. It may be possible to construct a large, square field on very flat land, but not quite so in sloping areas.

3. What area does the farmer think is suitable? This can limit field size and affect field shape. (See paper on site selection, this volume.)

4. How large an area does the family feel comfortable with trying out (especially for beginners)?

5. How large an area does the family think it can prepare and manage? (What does «manage» mean?) (See paper on feeding and maintenance, this volume.)

Some people say that a square field of 0.5-1 ha is the best size for rice fish culture. However, operations larger or smaller than this «ideal» size can also be very successful. Good preparation and good management are the keys to success, whatever the size.

A

Existing ricefield

B

Small dike between field and pond/trench; optional but useful if fish are stocked in the pond/trench before rice is transplanted

C

Side of pond trench; slope should depend on nature of soil, more gentle in sand, can be steeper in clay

D

Level of water in pond/trench prior to transplanting

E

«Lip» of existing field between pond/trench and dike, to prevent eroded material from filling the dike; usually 0.5-1 m wide

F

Side of dike; slope depends on nature of soil (gentle in sand); with top soil and grass, erosion is reduced

G

Maximum flood level; point G is most important; the dikes should be high enough so they cannot be submerged by floodwater

H

Plants/trees grown on top of dike

I

Keep excavated topsoil and sods for the outside of the dike; compact the soil during construction, as possible

Dikes

All dikes must be built safely higher than maximum flood levels. During construction, the dike should be raised high enough to allow for compaction and erosion.

In raising the dikes, an excavation usually results. This may as well occur inside the field, all other factors being equal. This way, a small pond or trench is formed, which serves as a refuge for the fish.

Refuges

A refuge is a pond, trench or low point in the rice-fish field. When the rest of the field is dry, fish can be held here. Under some conditions (see paper on fry nursing in rice-fish systems, this volume), the refuge may be stocked before rice is transplanted.

Having a refuge is usually advisable and may be necessary for success. Without it, fish have to be harvested before the field dries out or moved to a pond in a flooded area. A refuge of at least 50 cm depth is desirable. If the farmer wants to hold fish all year around, the refuge will probably have to be much deeper than this.

In some well-irrigated areas, a refuge may not be necessary. Some farmers find that digging a refuge increases water loss. This can happen in cases where poor soil (like sand) is covered by the top soil, which seals water in. Digging a trench may break this seal; it will reform, but this will take time. Manuring speeds up the process.

A refuge, when dug, is usually made at the lowest part of the field so that water and fish can easily collect there.

Some other factors governing size and arrangement of refuges:

1. How much rice-growing area is the family willing to sacrifice for the refuge? This may depend on their total rice-growing area or on the relative importance they give to rice and fish.

2. How much money or time and labour can the family invests? As with field size, this can be an important limit.

3. What kind of soil is involved? A narrow trench (say 1 m wide x 1 m deep) will fill in quickly in sandy soil, but may last well in clay. The refuge in sandy soil should be three or more times wider than its depth.

4. Topography will affect trench or pond configuration. Extensive peripheral trenches on sloping areas will occupy too much space since such a field will be narrow.

Consider these two fields, each of 16 mē area:

The narrow field has the greater perimeter to area ratio.

Some sample refuge layouts with comments

Plan Cross-section

Comments

Easy access for fish. Carrying excavated soil to dike can take time.

Easy access for fish. Best for large fields on very flat land. Can be expensive to build. Difficult entry for buffaloes.

Widely applicable on flat or sloping land, especially for plots of less than 0.5 ha (but can work for larger plots, too).

When trench is on low side of field on sloping land with porous soil, seepage can be a serious problem. Digging trench below ground level (rather than merely damming at ground level) and manuring can help. So can excavating the trench on the uphill side of the plot and sloping the plot toward the trench. This, however, can take a lot of work.

A common setup in rainfed northeast Thai systems. Small pond for refuge, in a system consisting of many plots on gently sloping land. Pond is usually at or near lowest part of field. Height of enclosing dike decreases as one goes uphill. This can help water catchment. Accommodates small-scale environmental variation, with little work. Farmers should be careful not to allow fish to have access to ponds when water is low.

Narrow, shallow trenches connected to refuges can be very helpful to fish trying to reach the refuges. One or two rows of rice may have to be sacrificed.

Drains

Usually, the field will need a drain so that excess water can be removed rapidly without eroding the dike. Inflow and especially outflow drains are advisable. Drains should be screened to prevent fish from escaping.

What material?

A bamboo, hollow log or pipe can be used, depending on availability. A screen should be placed at the point where the water enters. The screen can be a piece of fine netting or of flat metal full of nail holes. A little gravel scattered under the pipe will reduce dike erosion.

Such a drain is best for small fields (less than 1 000 mē) with limited flow (especially fields used to nurse hatchlings or small fry). Screens need to be checked every few hours for clogging any time water rises to pipe level and this can be a nuisance.

In most fields, the drain consists of a simple breach in the dike. This is screened by thin splints of bamboo or similar material, bound or nailed together.

Farmers in the rainier parts of northeast Thailand often use a li, a bamboo chute, set at a breach in the dike at the lowest part of the ricefield.

The li slopes up slightly and narrows. Below the narrow end, a jug-shaped basket or net bag is set. Hence, water runs out along the length at the li, but ultimately falls into a bag or basket which holds any fish washed out of the field. These can be eaten, sold or returned to the field for further growth.

Some farmers use a simpler version, by setting a net bag supported by sticks next to the outlet drain.

Outflow drain level

What depth of water is best for the rice in the field? What is the greatest depth it will tolerate? Set the drain in the dike somewhere between these two levels.

What drain capacity?

A small pipe does not drain a large field effectively. The farmer will have to make a guess as to how wide the drain should be, based on experience. It is better to have the drain a little too wide than too narrow.